Over the last 30 years Open Reduction Internal Fixation (ORIF) with Rigid Internal Fixation (RIF) has become accepted as the standard of care for treating many types of fractures helping patients painlessly return to pre-injury function earlier and more reliably than conventional treatment methods such as casting, bracing and interosseous or cerclage wiring. In addition, when properly applied RIF improves the reestablishment of pre-fracture anatomical bone alignment promoting more reliable infection free healing. Besides the proven benefits in trauma care, ORIF is an acceptable method of repositioning bones in elective procedures and repairing bones surgically cut or fractured when necessary to gain surgical access to perform a primary procedure. Such is the case in open-heart surgery where the sternum is surgically cut to gain access to cardiovascular structures contained within the chest wall. In such cases the sternum is surgically cut along the midline of the long axis of the bone separating the sternum and the associated rib cage in half sections left and right.
The standard method for reconstructing the surgically cut sternum is the placement of stainless steel wires circumferentially (cerclage) around the sternum segments and compressing together by twisting the wires tight to hold the surgically cut bone ends together approximating the pre-cut anatomical position of the sternum and chest wall. In most cases wire fixation has proven to be a successful and cost effective method of repairing the cut sternum with minimal reports of infection and non-union. The literature describes complication rates (infection and/or non-union) as high as 8%. Patients that incur complications, however, endure significant pain and resolving their issues has proven difficult, time consuming, and expensive.
Patients with certain underlying health issues are predisposed to complications. For instance, perhaps most significantly, certain cardiovascular patients with multiple health issues including, as examples, COPD, diabetes, and/or suppressed immune response that may delay or prevent healing, exhibit a propensity for post-operative infection, hardware failure and/or nonunion of the sternum. Other factors, such as age, poor diet, smoking, alcohol abuse and/or drug use, can also adversely affect healing. Many of these patients exhibit diseased bone that is weak and may lack cortical density and thickness.
Over the years, numerous attempts have been made improve a method for fixing the sternum, but most devices are designed to address the sternum after complications have arisen and are not intended to prevent complications by providing an improved primary solution. Furthermore, many of the commonly marketed products tend to be over engineered, complicated and time-consuming to implant. There are also a host of devices that do not appropriately address the complexities of the human anatomy and the demands such fixation must address in clinical applications. Those devices tend to offer no benefit over wire fixation and may lead to unexpected and unintended complications beyond what is known from wire fixation.
The sternum is a flat bone with a thin outer cortex (dense outer bone layer). Cortical density and thickness are important with screw fixation techniques as they provide resistance against pullout when screws are tightened as purchase is achieved by the threads compacting and resting in bone. Cortical density and thickness are also important factors in cerclage wire fixation as stability relies on wires compressing against the cortex to maintain secure fixation.
An implant construct must provide and maintain sufficient stabilization for a duration long enough to allow bone healing to occur. If healing does not occur within an acceptable timeframe hardware loosening often leading to hardware failure becomes an increasing risk. This principle also applies to sternum fixation. In the patient population prone to delayed healing and increased risk of complication, cerclage wire fixation may be contraindicated. In such cases, failure occurs due to broken or loosened wires. In some instances, such wire(s) loosens by cutting through the sternum cortex (commonly referred to as the “cheese grater effect”), which leads to mobility of the bone fragments, potential fracture of the sternum, and almost certain infection. Frequently when patients exhibit failed cerclage wire fixation, radical debridement of soft tissue and bone is necessary and reconstruction resembles more of a salvage mission.
Coughing, which is a very common post-operative occurrence, especially with patients with COPD or pneumonia, can cause high tensile forces on the repaired sternum, thus increasing the rate of failure of cerclage wire, as well.
Uncontrolled motion between two fractured bone fragments may also contribute an increased incidence for infection. As such, the fixation construct chosen must control motion under functional loading conditions to create a favorable healing situation. Opinions have varied over the years as to how much rigidity is desirable in a fixation construct. Historically, it was considered a treatment goal to create a motion-free interface between two bone fragments which was achieved by compressing the fractured or severed bone surfaces in direct opposition, eliminating all motion and encouraging direct healing without the formation of a callus. However, it has now been realized, through the passage of time and the gaining of valuable experience in this area, that the need for extreme rigidity, and thus the elimination of all motion in this situation, is not necessary nor the prevention of callus formation. In essence, it has been found that fixation constructs that are substantially more rigid than the bones they are holding can lead to a condition known as stress shielding that fosters poor bone quality and strength of healed bone and giving rise to potential secondary complications. Excessively strong implant constructs can also create stress risers that predispose bone to potential fracture or re-fracture. Load-sharing by implants is increasingly gaining favor as it is thought to promote healthier and stronger bone.
Another consideration is whether fixation implants can and should be left in the body long-term or permanently. There are many factors to consider such as patient age, the anatomical location of the implant, and the difficulty in removal. Generally, however, most surgeons prefer to leave fixation implants in vivo permanently and not perform a secondary procedure for removal whenever possible. Many cases of fixation implant removal result from patient complaints of discomfort, irritation, and palpability. An ideal implant design is one that can be left in permanently and causes little or no pain or discomfort to the patient during the healing phase and beyond.
The implant material is another major consideration in making the best implant
fixation choice. It is vitally important (for clear reasons) that the implant be biologically stable and not cause irritation or another undesirable reaction while in the body. Furthermore, consideration should be given to an implant's potential effect on diagnostic, imaging, monitoring and other therapeutic technologies necessary to care for post-operative patient care.
The speed and ease of installation are important considerations to make when choosing an implant fixation construct. Cardiovascular surgeons are not orthopedists and therefore not routinely familiar with drills, screwdrivers and other “bone carpentry” tools. Many sternum closure devices currently offered require such items as they are based on orthopaedic plate and screw technology. These devices typically require multiple instruments, have many individual parts, and take an excessive amount of time to install adding additional time and cost to the surgery.
The speed and ease of implant removal are also critical factors when choosing a fixation implant construct, especially in the case of a target sternum whereby emergency surgical re-access may be required should the patient incur a life-threatening health event necessitating surgical reentry of the chest wall. If a device requires special instruments to remove or has become biologically imbedded in the soft tissues and/or bone, valuable time can be lost dealing with locating removal instrumentation and exposing and removing the implants.
Additionally, the cost of an implant device construct must be reasonable and not add significantly to the overall cost of performing surgery. In the case of the sternum cerclage wire fixation, the material cost of surgical wire is insignificant. Plate and screw constructs for sternal closure range in price but easily can cost $3,000 to $5,000 per device. In addition, there are disposable components, such as drill bits, etc., that add to the cost of surgery. Most sternum-plating sets are configured as reusable trays containing an assortment of implants and reusable instruments requiring sterilization, cleaning, and restocking between each use requiring additional costs and labor.
Current sternal fixation devices include rigid-plate solutions with elaborate locking screws (where the screws simultaneously thread into the plate and sternum). Implants, such as those available from Synthes, comprise of two or three of the plates consisting of left and right segments joined together by a quick release “grenade” pin mechanism. These plates are spaced and implanted along the anterior facing sternal surface midline straddling the saw cut with screws inserted into the sternum on both sides of the cut. If emergency re-access becomes necessary, the operator must remove the pins and separate the sternum and associated rib attachments left and right giving immediate access through the chest wall. Uncoupling the plate only uncouples the bone when the bone remains unhealed. If reentry is attempted after the soft tissue and bones have healed, simply removing a pin will not provide immediate re-access. Further complicating access during revision surgery is the positioning of the bulky metal implant directly over the desired placement for the saw cut needed to open a partially or fully healed sternum. Such screw-secured implants are also very time-consuming to implant and costly to produce. Their excessively rigid construction results in stress-shielding and detrimental bone loss and possibly delayed and or poor healing.
In another variation of a prior device, reduced stress shielding has been provided through the utilization of braided cables through sterna-positioned cannulated metallic grommets. Unfortunately, though, this alternative still requires excessive operating time and a skill-dependent implantation procedure. The cable is laced along the sternum like laces on a shoe and tightened with a special cable crimping instrument. The process for installation is too cumbersome and time consuming and getting the bone segments back into anatomical position has proven too difficult for widespread, reliable use.
Self locking ties, similar to “zip ties” placed around the sternum through the intercostal spaces of the sternum provide an improvement in simplicity, however, provide no better fracture immobilization than cerclage wire. The zip-tie fixation method disregards the significant forces loaded on the sternum and is not an adequate solution for, in particular, at-risk patients. Therefore, locking ties only appear to offer a potentially more convenient way to achieve the same benefits as cerclage wiring and may contribute to complications resulting from unsatisfactory mechanical characteristics when used in such an application.
Other devices attempting to solve the sternal closure method include a mechanical clamp that cleats around the sternum passing through the intercostal spaces. When used in series, these metallic clamps compress the sternum together. The clamps are large, excessively rigid and frequently uncomfortable and irritating to the patient frequently necessitating post-operative removal, as well as comparatively costly.
The other identified competitive offerings seem to follow a plate-and-screw approach to fixing the sternum, typically with cuttable struts across the central section facilitating removal. None of them appear to offer significant benefits over each one another. Due to the significant forces that act on the sternum under normal and extreme functional loading all present similar risks of post-operative complication.
A need thus exists for an inexpensive, implantable, load-sharing sternal fixation device that is easy to implant, minimizes disruption to the surrounding soft tissues, and that allows simple removal to re-access to the chest cavity by conventional methods. To date, the sternum fixation industry has yet to provide such a beneficial alternative to the current devices described above.